Piezoelectric transducer and method for producing same



April 20, 1954 R. ADLER Re. 23,813

PIEzoELEcTRIc TRANsDucER AND METHOD FOR PRonucmG SAME Original Filed Dec. 26, 1947 2 Sheets-Sheet 1 ROBERT ADLER mmvron l-ns ATTORNEY.

April 20, 1954 R. ADLER Re. l23,813

PIEZOELECTRIC TRANSDUCER AND METHOD FOR PRODUCING SAME original Filed Deo. 26, 1947 2 sheets-snee: V2

F|G.7 FIG.8

Polarization so FI G 9 F I G. 11

ROBERT ADLER INVENTOR.

HIS ATTORNEY.

Reiseued lApr. 20, 1954 Re. 23,813l

`PIEZOELECTRIC TRANSDUCER AND METHOD FOR PRODUCING SAME Robert Adler, Northfield, Ill., assigner to Zenith Radio Corporation, a corporation of Illinois Original No. 2,540,412, dated February 6, 1951,

Serial No. 793,892, December 26, 1947. Application for reissue February 3, 1953, Serial No.

y 28 Claims. (Cl. 179-100.41)

. Matter enclosed in heavy brackets appears in the original patent but forms no part of this reissue specification; matter printed in italics indicates the additions made by reissue.

This invention relates to piezo-electric transducers and to methods for producing such transducers.` It is a primary object of the invention of artiiicial piezo-electric transducers is specifically disclosed and claimed in the copending application of Walter L. Cherry, Jr., Serial Number 770,163, illed August 22, 1947, now Patent No. 2,538,554, issued January 16, 1951, for Piezo- Electric Transducers. and which application is assigned to the same assignee as the present application.

Such articial piezo-electric transducers as are specifically disclosed in the aforementioned [copending application] Cherry patent are particularly luseful for high frequency applications. However, for optimum coupling, the direction of mechanical stress must be identical with that of the piezo-electric axis and that of the alternating field; consequently transducers of this type are not readily applicable to audio frequency devices. The minimum frequency at which such transducers may be operated is determined by the maximum capacitance, or electrical compliance, commensurate with practical values of mechanical compliance. If the mechanical compliance is made large enough to suit practical requirements by making the cross-sectional area small, the associated capacitance is too small for audio frequency applications. If, on the. other hand, a practical value of capacitance is obtained by making the cross-sectional area large, the mechanical compliance becomes inconveniently small. Y,

-It is a particular object of this invention to provide improved artliicial piezo-electric transducers which are suitable for operation at audio frequencies.

In accordance with the invention, ithas been found that the capacitance or electrical compliance of the artificial transducers may be increased while maintaining a practical value of mechanical compliance by varying the direction of the piezo-electric axis within a single-unit transducer. It is an important object of this invention, therefore, to provide methods for producing a single-unit piezo-electric transducer in which the direction of the piezo-electric polycrystalline aggregate axis varies at a non-uniform rate from portion toportion along at least one maior dimension of the aggregate.

It is a further object of the invention to provide an artificial piezo-electric transducer suit- 5 able for operation at audio frequencies by virtue 'of an increased electrical compliance resulting from such non-uniform variation of the direction of the piezo-electric axis.

Yet another Vobject of the invention is to provide a novel process for inducing a piezo-electric eifect in a preformed polycrystalline aggregate body, the direction of the piezo-electric axis varying at a non-uniform rate from portion to portion along at least one'major dimension of such.

body in accordance with a predetermined pattern.

Frequent applications of piezo-electric transducers in the audio frequency range are made in phonograph pickups, microphones. and the like. In such applications, it is necessary to providea transducer which has permanent piezoelectric properties with respect to bending stress. It is an important object of the present invention to provide a piezo-electric polycrystalline aggregate transducer which is suitable for use as a phonograph 4pickup or the like, and which is rugged. inexpensive, and durable.

In the specification and claims. the term direction," as applied to the piezo-electric axis or to the polarizing field, includes the concept of sense; therefore the phrase piezo-electric axes having different directions includes piezo-electric axes of opposite sense.

The term unidirectional,. as applied to polarizing fields, is employed with reference to time,

and ynot with reference to space. Hence a unidirectional polarizing fleld is one which is pro- -duced by a unidirectional potential difference.

The terminology "polycrystalline aggregate is employed to ,connote' a unitary structure comprising a large number of minute crystals. 'I'he term ceramic" necessarily implies such a structure.

The features of the invention which are believed to be novel are set forth particularly in the appended claims. The invention may more readily be understood. however, by reference to the following description taken in connection with the accompanyingpdrawings, in which like reference numerals indicate like `elements, and in which: Figure 1 is a schematic representation of a theoretical means for attaining the objects set forth above.

Figure 2 is a schematic representation of a Figure 13 is a schematic representation of a further embodiment of the invention.

Referring now to Figure 1, there is shown schematically a slab 20 of piezo-electric material, the direction of the piezo-electric axis being assumed. to be longitudinal or substantially parallel to the extended surface of the s lab throughout substantially its entire cross-sectional area. It is seen that the mechanical compliance in the longitudinal direction may well be high enough to suit practical requirements, since the cross-sectional area is small relative to the length, but the capacitance between electrodes located on the small surfaces 2| and 22 separated by the full length of the piece 2li is inconveniently small. I-n order to increase the capacitance or electrical compliance, it is assumed that slab 2U is theoretically divided into n slices 23-32 of equal length; the dotted lines indicate any desired number of intermediate slices. The capacitance between the imaginary transverse surfaces of each slice is therefore n times that between the two end surfaces 2| and 22 of slab 20, while the mechanical compliance between surfaces 2| and 22 remains unchanged. Furthermore, by interconnecting alternate transverse faces of the imaginary slices 23-32, as shown, and by connecting each set of alternate transverse faces to one of a pair of terminals 33 and 34, the total capacitance between surfaces 2| and 22 is made equivalent to n times the capacitance between the end faces of an individual imaginary slice 23; consequently the effective capacitance between surfaces 2i and 22 is increased by a fractor of n2. Ideally such a condition may be accomplished by polarizing adjacent slices 23-32 along the same axis but in opposite directions. Such a condition is shown schematically by the arrows which indicate the direction of polarization, and hence the direction of the piezo-electric axis, in each of the imaginary slices.

It has been found that permanent piezo-electric properties may be induced in such polycrystalline aggregate, and furthermore, that the direction of the piezo-electric axis induced in such aggregates is identical to that of the electrostatic ux setv up by the polarizing fields. Consequently, such a condition as shown schematically in Figure 1 might be accomplished if suitable unidirectional polarizing ilelds could be set up between the transverse faces of each of the imaginary slices 23--32. Obviously, such a situation is impractical, since no electrodes exist on the imaginary transverse surfaces between the slices.

As a practical approach to the optimum condition shown in Figure 1, there is shown in Figure 2a slab 40 of suitable polycrystallineaaggregate. such as a ceramic comprising barium titanate or barium strontium titanate mixed with a small amount of a. glass forming oxide and fired to vitriiication in accordance with the aforementioned [copending application] Cherry patent,

on opposite faces 4| and 42 of which have been disposed a plurality of electrical terminals 43- 54- F01' purpose of illustration, twelve terminals have been shown; however, other numbers of terminals may be employed. Alternate pairs of opposite terminals 43, 44, 41, 48, 5|, 52 and 45, 46, 49, 50, 53, 54 are interconnected and brought outto a pair of input terminals 55 and 56 respectively. When a unidirectional potential difference is applied between input terminals 55 and 56, adjacent portions of slab 4|! are polarized in substantially opposite directions.

Thus, the transducer of Figure 2 comprises asolid polycrystalline aggregate in which a large number of minute individual crystals are bonded together, the aggregate having permanent piezoelectric properties, and the direction of the piezoelectric axis varying at a non-uniform rate from portion to portion along at least one major dimension-the length dimensionof the aggregate. Furthermore, the distribution of the direction of the piezo-electric laxis within the aggregate is periodic, comprising a series of identical pairs of adjacentoppositely polarized portions.

In order more fully to show and explain such polarization, there is shown in Figure 3 a detail view of a portion of slab 40 of Figure 2. The electrostatic ux distribution, and hence the direction of polarization, is assumed, for purposes of explanation, to have a sense from to If then, it is assumed that input terminal 55 is the positive terminal and input terminal 56' is the nega-tive terminal, the electrostatic field, and hence the direction of the piezo-electric axis, differs abruptly from portion to portion as schematically shown in Figure 3.

It is seen that the embodiment shown and described in conjunction with Figures 2 and 3 is only an approximation of the' ideal shown and described in conjunction with Figure l. Since those portions of the slab 40 of ceramic mate-` rial which lie between opposite pairs of terminals, 41 and 48, for example, carry little or no electrostatic ux, these portions may be regarded as being waste portions. In order to effect a compromise between the desired eiective increase in electrical compliance and a minimum of waste material, it has been found that the width of the individual terminals 43-54 should b e of the order of the thickness oi' the slab 40. and the distance between successive terminals, 41 and 49, for example, should be of the order of twice the thickness of the slab 40. These proportions are the result of practical experiment and are intended in no sense to be construed as limitations, as other proportions may be used with varying degrees of eiilciency.

In certain audio frequency applications, it is desirable to employ a piezo-electric element which is sensitive to bending. Two elements, each formed in accordance with the method shown and described in connection with Figure 2, may be fastened together, by cement or in some other suitable manner, to provide such a dimorphic element. Such a configuration is shown in Figure 4, wherein a pair of elements 60 and 6| are fastened together to form a composite body 62. In operation one element lill is instantaneously subjected totensile stress while the other element 6I is instantaneously subjected to compressive stress, or vice versa. Therefore, care must be taken that elements GII and 6l are assembled and electrically connected in such a manner that the useful outputs of the individual elements are additive in the composite transducer 62.

` provided by properly applying polarizing elds to a single unitary polycrystalline aggregate body. Such an element is shown in Figure 5, wherein a unitary sla'b 18 of barium titanate or other suitable aggregate is provided with a plurality of electrical terminals 1I-82 similarly disposed along oppositeI faces 83 and 84; Alternate terminals 1|, 15, and 18 on face 83 and alternate terminals 14, 18, and 82 on face 84 are interconnected and brought out to an input terminal 85,

and the remainder of the terminals 12, 13, 18, 11,

. 88, and 8| are interconnected and brought out to a second input terminal 88.

By applying a unidirectional potential differ'- ence between input terminals85 and 88, unidirectional polarizing fields are applied between succsive terminals, 1| and 13, for example, on each'face 83, 84 of slab 10. Furthermore, each terminal-1I, for example-is oppositely polarized from the terminal-12, for example-Which is directly opposite therefrom, and the polarizing iields between opposite pairs of successive terminals-1|, 13 and 12, 14 for exampleare opposite in sense. By these means, the direction or sense of the piezo-electric axis is made to diier abruptly from portion to portion throughout the single piece 18 in a manner similar to the piezo-electric axis distribution in a composite dimorphic element such as that shown in Figure 4.

l After maintaining the polarizing elds for a suilicient period of time at least to approach saturation of the piezo-electric eii'ect, such elds are` removed, and the connections of terminals 1 |82 are changed to insure additive outputs in response to bending. Slab 18 with terminals 1 |-82 reconnected .for proper output at.. terminals 85 and 88 is shown in Figure 6, in which alternate pairs of opposite terminals 1I,'12, 15, 18, 18,- and 88, and 18, 14, 11, 18, 8|, and 82 are respectively interconnected. Such an interconnection is desirable in order to minimize the shunt capacitance between output terminals 85 and 88 which eiectively reduces the useful output of such a transducer. The embodiment of Figures 5 and 6 is speciiically claimed in the copending divisional application of Walter L. Cherry, Jr., Serial No.

159,613, filed May 2, 1950, now Patent No. 2,540,- 187, issued February 6, 1951, for Piezo-Electric Transducer and Method for Producing Same, which divisional application is assigned to the present assignee.

Figure 'I shows an alternative method of producing essentiallyr the same result while greatly reducing any shunt capacitance across the output terminals. In this embodiment, a pair of polycrystalline aggregate bodies 98 and 8| are fastened together by cementing means 82 of low dielectric constant thereby to form a composite body. Cementing means 82 may comprise a layer of cementing material of low dielectric constant, although the desired results may be achieved by glazing slabs 88 and 8| and firing with the glazed sides in contact, by ring a sandwich of three ceramic slabs, or by other suitable means. Opposite faces of the composite body are provided with' a plurality of similarly disposed electrical terminals 83-88 and 88-I84, alternate pairs of op- Y posite terminals 93, 98, 95, |8I, 91, |03, and 94, |00,

. 88, |82, 88, |84 being interconnected and brought out to a pair of input terminals |85 and |88. re-

' spectively. After polarizing in the usual manner, the terminal connections on one face of the composite body are reversed to insure additive outputs in response to bending, as shown in Figure 8, and terminals and |88 become output terminals. Although opposite terminals (83 and 98, for example) have opposite polarities associated therewith, the thin layer of cementing materiall 82 so reduces the effective capacitance shunting the output that satisfactory operation is insured. The embodiment of Figures 7 and 8 is specifically claimed in the copending divisional application lof Alexander Ellett, Serial No. 159,- 620, iiled May 2, 1950, now Patent No. 2,540,194, issued February 6, 1951, for Piezo-Electric Transducer and Method for Producing Same, assigned to the present assignee.

While the embodiment shown in Figures 7 and 8 is formed by' cementing bodies 90 and 8| together lbefore polarization it is to be understood that the individual bodies 88 and 9| may be polarized separately and then rmly united in two such bodies are cemented or otherwise fastened together to produce a composite body such as that shown in Figure 8.

It is also to be understood that, although pairs of terminals 93-I84 have been shown connected in parallel, pairs of terminals may be connected in series in certain applications in which such connection --is desirable.

As a variant of this embodiment, two slabs of polycrystalline aggregate such as slab 88 in Figure 9 may be provided each with electrical terminals differently disposed in such a way that. after polarization, such slabs may be fastened together in a manner similar to that shown and described in conjunction with Figure 4. In this way, objectionable shunt capacitance between the output terminals may be avoided Without the use of low dielectric constant cementing means.

As a further variant, the composite body shown in Figures 7 and 8 may be polarized with the terminals 93-I 04 connected as in Figure 8, in which case the terminals 83|84 are reconnected as shown in Figure 7 to insure additive outputs. In this manner, the shunt capacitance across the output is minimized by interconnecting opposite terminals.

There is shown in Figure 10 a phonograph pickup I|8 constructed in accordance with the invention. The terminals III and |,I2 are shown in the form of a pair of intermeshing combs for efficient polarization, and may be of silver paint or other suitable material applied by silk screen, vacuum evaporation, or other suitable process. In a practical application, the terminals applied on the back side (not shown) of element I I8 may be made in the form of a mirror image of those applied to the front side in order to minimize the undesired capacitance shunting the output. One end I I3 of transducer III) is firmly clamped in a bracket II4, and the other end I I5 is provided with a. rigid extension I I8 so constructed and arranged that the edges I |1 and I I 8 of transducer I I8 and extension I I8 all converge to a point |I8. Lateral motion at point I I8, which may be translated from undulations of a record disc by means sans of a stylus I I9' secured to extension ||6 at point IIS, then results in corresponding electrical output between terminals and ||2; by making transducer trapezoidal in shape, the conditions of a uniform-stress beam are approached.

'I'here is shown in Figure 11 a plan view of a further embodiment |20 of the dimorphic type which might be applied in microphones, speakers, or the like. Such an embodiment as shown comprises a pair of circular polycrystalline aggregate bodies cemented together, each of which is suplplied with a pair of interleaved spiral-shaped electrical terminals |2| and |22. In such an embodiment,polarization is effected in the usual manner, with the result that the direction of the piezo-electric axis in the finished product is at all points substantially radial, and the sense of the piezo-electric axis is different between successive turns of the terminals |2| and |22.

Figure 12 is a perspective view of a cylindrical radiator |30 which is adapted to vibrate radially at a frequency determined by the circumference; such a radiator comprises an element of the type shown in Figure which has been preformed in cylindrical form. After polarization in the manner set forth above, alternating electrical input to terminals |3| and |32 causes radial vibration of element |30.

The present invention also includes other conflgurations which can never be duplicated by natural piezo-electric crystals, since it is possible to polarize a preformed ceramic body of any shape in any manner consistent with an electrostatic flux pattern. As a further embodiment of the invention, transducers having permanent piezo-electric properties with respect to composite vibration may be produced. The direction of the piezo-electric axis is different in different portions of such a transducer; furthermore, the direction of the piezo-electric axis changes abruptly from portion to portion. There is shown in Figure i3 a transducer of this nature.

In Figure 13, a slab |40 of suitable polycrystalline aggregate, which has been polarized in a manner identical with that shown and described in conjunction with Figure 2, is provided with means for applying an electrostatic field, such means as shown taking the form of a pair of' electrodes |4| and |42 applied to respective ends of transducer |40. Upon application of an alternating potential difference between electrodes |4| and |42, an alternating field is produced in body |40, the space relation between the fleld and the piezo-electric axis being different in different parts of the transducer. Consequently, stresses are produced parallel to the direction of the field, but of opposite signs between successive pairs of polarizing terminals (44, 46 and 48, 48, for example) This results in resonant composite vibration of body |40 at a high frequency, adjacent parts vibrating in opposite phase and at a, frequency such that the distance between adjacent polarizing terminals (44 and 46, for example) is substantially a half-wavelength.

Such a transducer is eflicient at high frequency, since the high frequency resonant vibration is attained without the accompanying sacrifice of electromechanical coupling associated with harmonic vibration of natural piezo-electric materials. Consequently, high frequency vibration may be induced in a. relatively large transducer.

In all of the embodiments of the invention, permanent piezo-electric properties are imparted to a solid polycrystalline aggregate in such a manner that the direction of the piezo-electric tion along any radial dimension, while in the embodiment of Figure 12. the direction of the piezo-electric axis varies at a non-uniform rate from portion to portion along the circumferential dimension of the body. I

While there have been shown and described certain preferred embodiments of the invention, it will be understood that numerous variations f and modifications may be made, and it is contemplated, in the appended claims. to cover all such variations and modifications as fall within the true spirit and scope of the invention.

I claim:

l. The process of producing a piezo-electric effect in a solid polycrystalline aggregate in which individual crystals are bonded together, said aggregate having an extended surface area which is large relative to its transverse cross-sectional area, said process including producing a unidirectional electrostatic polarizing field which, throughout said cross-sectional area, is substantially parallel to said extended surface and substantially perpendicular to said cross-sectional area, the direction of [which varies] said field varying in a non-uniform rate from portion to portion along at least one major surface dimension of said aggregate, and maintaining said field for a substantial period of time at least approaching that necessary for saturation of said eect.

2. The procs of producing a piezo-electric effect in a solid aggregate in which individual crystals of barium titanate are bonded together, said aggregate having an extended surface area which is large relative to its transverse cross-sectional area, said process including producing a unidirectional electrostatic polarizing field which, throughout said cross-sectional area, is substan# tially parallel to said extended surface and .sub-` stantially perpendicular to said cross-sectional area, the direction of [which varies] said field varying at a non-uniform rate from portion to portion along at least one major surface dimension of said aggregate, and maintaining said field for a substantial period of time at least approaching that necessary for saturation of said effect.

3. The. process of producing a permanently piezo-electric ceramic body, said process comprising heating a thorough mixture of polycrystalline material and ceramic binder to a temperature suflicient to bond the individual crystals of said material together thereby to form a solid polycrystalline aggregate, said aggregate having an extended surface area which is large relative to its transverse cross-sectional area, thereafter producing a unidirectional electrostatic polarizing field which, throughout said cross-sectional area, is substantially parallel to said extended surface and substantially perpendicular to said crosssectlonal area, the direction of [which varies] said field varying at a non-uniform rate from portion to portion along at least one major surface dimension of said aggregate. and maintaining said eld for a. period of time sufllcient at least to approach saturation of thc piezo-electric effect in said aggregate.

4. The process of producing a permanently piezo-electric ceramic body, said process comprising heating a' thorough mixture of polycrystalline material and ceramic binder to a temperature sumcient to bond the individual crystals of said material together to form a solid polycrystalline aggregate, similarly disposing along each of two opposite faces of said aggregate a plurality of electrical terminals, producing unidirectional polarizing fields substantially parallel to said faces throughout said body and having substantially opposite directions between successive pairs of terminals on each of said faces, and maintaining said fields for a period of time at least approachthe piezoof time at least to approach saturation of the piezo-electric effect in said aggregate.

6. A transducing element including a solid ceramic polycrystalline aggregate having an extended surface area large relative to its transverse cross-sectional area and in which a large number of minute individual crystals are bonded together, said aggregate having permanent piezo-electric properties, and the direction. of the piezo-electric axis, throughout said element, being substantially parallel to said extended surface and substantially perpendicular to said cross-sectional area and varying at a non-uniform kcrate from portion to portion along at least one major swrface dimension of said aggregate. I

7. A transducing element including a solid polycrystalline aggregate having an extended surface area which is large relative to its transverse crosssectional area, said aggregate including a large number of minute individual crystals of barium titanate bonded together :by a ceramic binder,

said aggregate possessing permanent piezo-l electric properties and the direction of the piezoelectric axis, throughout said cross-sectional area, being substantially parallel to said extended surface and substantially perpendicular to said crosssectional area and varying at a non-uniform rate from portion to portion along at least one major surface dimension of said aggregate.

8. A transducing element including a solid polycrystalline aggregate having an extended surface area which is large relative to its transverse crosssectional area, said aggregate including a large number of minute individual crystals of .barium strontium titanate bonded together by a ceramic binder, said aggregate possessing permanent piezo-electric properties and the direction of the piezo-electric axis, throughout said cross-sectional' area, being substantially parallel to said ertended sur/ace and substantially perpendicular to said cross-sectional area and varying at a nonunitorm rate from portion to portion along at least one major surface dimension of said aggregate.

9. A solid polycrystalline aggregate body having a plurality of electrical terminals similarly dis- ,10 posed along opposite faces thereof, said body having permanent piezo-electric properties and the direction of the piezo-electric axis being substantially parallel to said faces throughout said body and substantially opposite between successive pairs of terminals on each of said faces.

10. A ceramic element having permanent piezoelectric properties, said element including a solid polycrystalline aggregate, a plurality of electrical terminals similarly disposed along each of two opposite faces of said aggregate, the direction of the piezo-electric axis substantially parallel to said faces throughout said element and substantially opposite between successive pairs of terminals on each of said faces, and the direction of the piezo-electric axis between successive pairs of terminals on one of said faces being the same as the direction of the piezo-electric axis between corresponding pairs of terminals on the other of said faces.

11. A ceramic transducing element having permanent piezo-electric properties, said element including a pair of similar polycrystalline aggregate bodies, and'a plurality of parallel electrical terminals similarly disposed along each of two opposite faces of each of said bodies. the direction of the piezo-electric axis being substantially parallel to said faces throughout said bodies and substantially opposite between successive pairs of terminals on each of said faces,.and a pair of corresponding terminal-bearing faces of said bodies being firmly united by cementing means.

12. A ceramic element having permanent piezo-electric properties, said element including a pair of solid polycrystalline aggregate'bodies fastened together thereby to form a composite body, and a plurality of electrical terminals disposed along each of two opposite fades of said composite body, the direction of the piezo-electric axis being substantially parallel to said faces throughout said bodies and substantially opposite between successive pairs of terminals on each of said faces.

13. A ceramic element having permanent piezoelectric properties, said element including a pair of solid polycrystalline aggregate bodies fastened together thereby to form a composite body, said aggregate including individual crystals of barium titanate bonded together with a ceramic binder, a plurality of electrical terminals disposed along -each of two opposite faces of said composite body. the direction o f the piezo-electric axis being substantially parallel to said faces throughout the bodies and substantially opposite between successive pairs of terminals on each of said faces.

14. A ceramic element having permanent piezoelectric properties, said element including a pair of solid polycrystalline aggregate bodies firmly united to form a composite body, a pair of interleaved spiral-shaped electrical terminals disposed on each of the opposite facesof said composite body, the direction of the piezo-electric axis being always substantially radial and being different between successive turns of said spiralshaped terminals.

15. A hollow cylindrical polycrystalline aggregate body having permanentpiezo-electric properties and having a plurality of parallel axial electrical terminals disposed along the outer surface thereof, the direction of the piezo-electric axis being at all points substantially circumferential and being substantially opposite between successive pairs of said terminals.

16. Aceramic element having permanent piezoelectric properties, said element including a solid polycrystalline aggregate body having an extended surface area large relative to its transverse cross-sectional area, said aggregate having permanent piezo-electric properties with the direction of the piezo-electric axis throughout said aggregate being substantially parallel to said extended surface and substantially perpendicular to said cross-sectional area, and in different portions of which the direction of the piezo-electric axis is different, and means for applying to said body an electrostatic field, the space relation between said eld and said piezo-electric axis being diierent in different parts of said element, whereby stresses of opposite sign in said different parts are simultaneously produced in response to said field.

1'?. A pickup element for a phonograph or the like, said velement comprising a permanently piezo-electric body of barium titanate individual crystals of which are bonded together by a ceramic binder, a plurality of parallel electrical terminals disposed along opposite faces of said body, the terminal disposition on one of said faces being substantially a mirror image of the terminal d-isposition'on the other of said faces, the direction of the piezo-electric axis being substantially parallel to said faces substantially throughout said body and substantially opposite between successive pairs of terminals on each of said faces, and means for bending said composite body in responsel to undulations in a record disk.

18. A transducing element having electrical terminals and including a solid ceramic polycrystalline aggregate in which alarge number 'of minute individual crystals are bonded together, said aggregate having an extended surface area which is large relative to its transverse cross-sectional area, said aggregate having permanent piezo-electric properties, and the direction of the piezo-electric axis, throughout said cross-sectional area, being substantially parallel to said extended surface and substantially perpendicular to said cross-sectional area and being substantially opposite in adjacent portions pf said aggregate.

19. A transducing element having electrical terminals and including 4a solid polycrystalline aggregate having an extended surface area which is large relative to its transverse cross-sectional area, said aggregate including a large number of minute individual crystals of barium titanate bonded together by a ceramic binder, said aggregate possessing permanent piezo-electric properties and the direction of the piezo-electric axis, throughout said element, being substantially parallel to said extended surface and substantially perpendicular to said cross-sectional area and being substantially opposite in adjacent portions of said aggregate.

20. A transducing element having electrical terminals and including a solid polycrystalline aggregate having an extended surface area which is large relative to its transverse cross-sectional area, said aggregate including a large number of minute individual crystals of barium strontium titanate bonded together by a ceramic binder, said aggregate possessing permanent piezo-electric properties and the direction of the Piezo-electric axis, throughout said element, being substantially parallel to said extended surface and substantially perpendicular to said cross-sectional area and 12 number of minute individual crystals are bonded together, said aggregate having an extended surface area which is large relative to its transverse cross-sectional area and having permanent piezoelectric properties, said slab comprising atleast a pair of adjacent elemental portions along one face thereof, the direction of the piezo-electric axis in each of said portions, throughout said crosssectional area, being substantially parallel to said extended surface and substantially perpendicular to said cross-sectional area and the direction of the piezo-electric axis in one of said portions being substantially opposite to the direction of the piezo-electric axis in the other of said portions.

22. A transducing element including a slab of solid polycrystalline aggregate in which a large number of minute individual crystals are bonded together, said aggregate having an extended surface area which is large relative to its transverse cross-sectional area and having permanent piezoelectric properties, said slab comprising a plurality of elemental portions along one face thereof, the direction of the piezo-electric axis in each of said portions, throughout said cross-sectional area, being substantially parallel to said extended surface and substantially perpendicular to said cross-sectional area, the directions of the piezoelectric axis in alternate ones of said portionsbeing substantially identical, and the direction of the pizca-electric axis in adjacent portions being substantially opposite.

23. A transducing element including a slab of solid polycrystalline aggregate in which a, large number of minute individual crystals are bonded together, said aggregate having an extended surface area which is large relative to its transverse cross-sectional area, and having permanent piezoelectric properties, said slab having a pair of opbeing substantially opposite in adjacent portions positely disposed faces, each of said faces comprising a plurality of elemental portions, the direction of the piezo-electric axis in each of said portions, throughout said cross-sectional area, being substantially parallel to said extended surface and substantially perpendicular to said crosssectional area, the directions of the piezo-electric axis in alternate ones of said portions on each face being substantially identical, and the direction of the piezo-electric axis in each of said portions being substantially opposite to the direction of the piezo-electric axis 4in any adjacent portion of the same face.

24. A transducing element including a solid polycrystalline aggregate in which a large number of minute individual crystals are bonded together, said aggregate having an extended surface area which is large relative to its transverse cross-sectional area and having permanent piezoelectric properties, and the direction of the piezoelectric axis, throughout said element, being substantially parallel to said extended surface and substantially perpendicular to said crosssectional area and differing abruptly from portion to portion in said aggregate.

25. A transducing element including a solid polycrystalline aggregate in which a large number of minute individual crystals are bonded together, said aggregate having an extended surface area which is large relative to its transverse cross-sectional area and having permanent piezoelectric properties, and the direction of the piezoelectric axis, throughout said element, being subalong one dimension and differing continuously from portion to portion along another dimension of said aggregate.

26. A transducing element including a solid polycrystalline aggregate in which a large number of minute individual crystals are bonded together, said aggregate having an extended surface area which is large relative to its transverse cross-sectional area and having permanent piezoelectric properties, and the direction of the piezoelectric axis, throughout said element, being substantially parallel to said extended surface and substantially perpendicular to said cross-sectional area, the direction of the piezo-electric axis differing abruptly from portion to portion along one dimension and differing smoothly and continuously from portion to portion along another dimension of said aggregate, said other dimension being substantially perpendicular to said one dimension.

27. A unitary piezo-electric ceramic transducer having an extended surface area large relative to its transverse cross-sectional area and having adjacent [oppositely polarized] portions longitudinally polarized, throughout said cross-sectional area, in opposite directions substantially parallel to said extended surface and substantially perpendicular to said cross-sectional area.

28. A transducing element including a solid polycrystalline aggregate in which a large num ber of minute individual crystals are bonded together, said aggregate having an extended surface area which is large relative to its transverse cross-sectional area and having permanent piezoelectric properties, the direction of the piezoelectric axis, throughout said element, being substantially parallel to said extended surface and References Cited in the le of this patent or the original patent UNITED STATES PATENTS Number Name Date 2,242,756 Pope May 20, 1941 2,486,560 Gray Nov. 1, 1949 OTHER REFERENCES Roberts, Physical Review, vol. 71, pages 890- 895, June 15, 1947. 

